A typical digital wireless communications system consists of a transmitter, a receiver and communication channels wirelessly connecting the receiver to the transmitter. At the transmitter, an information bit sequence is coded and transformed into an analog waveform centered at baseband frequency, which is subsequently up-converted to be centered at a higher frequency in order to be sent over one of a number of predetermined radio frequency (RF) channels. At a given receiver, the RF received signal in the desired channel is down-converted to one or more successive intermediate frequencies (IF) before being converted to baseband frequency. The resulting analog baseband signal is then passed through an analog-to-digital (A/D) converter. The resulting digital baseband signal is used as input to a demodulator, a detector, a decoder, and a synchronization and tracking algorithm to recover the original information bit sequence.
In standard digital receivers, external analog local oscillators (LOs) and bandpass filters are used to implement each IF down-conversion stage, including the final conversion to baseband frequency. Furthermore, in one of these IF stages, an additional tunable LO is used to select the desired signal channel.
It has been found that, if the carrier frequency of the analog IF signal entering the baseband conversion stage, denoted as f.sub.c, is related to the sampling frequency f.sub.s of the digital signal processing (DSP) receiver by the following expression, called quadrature sampling condition, ##EQU1## EQU for m an integer
then the analog IF signal can be directly converted to baseband. Thus, the quadrature sampling method allows elimination of an external LO which would have been needed to further down-convert the IF frequency to baseband.
However, since the sampling frequency f.sub.s is typically fixed for a given receiver, the frequency f.sub.c of an incoming analog IF signal must be chosen to satisfy the quadrature sampling condition. This requires the use of a tunable external LO to tune to the desired signal channel and to translate the carrier frequency of a received analog signal to an IF carrier frequency that satisfies the quadrature sampling condition. Therefore, in practice, the quadrature sampling method does not eliminate the use of at least one tunable external LO.
The present invention discloses a method and a system for performing frequency selection and down-conversion for digital radios, modems and receivers without the need for an external tunable LO. Moreover, in the present invention, the frequency selection and down-conversion are performed digitally, for example, by a digital signal processor. If an A/D converter with sufficient bandwidth is used to provide the input to the digital receiver, then a front-end fixed down-conversion to an arbitrary frequency f.sub.c is not needed. Thus, with the present invention, all of the external analog LOs that are typically used prior to the A/D conversion of the received analog signal are eliminated.